This application is related to U.S. patent application Ser. No. 871,367 filed Jan. 23, 1978 by Hard and U.S. patent application Ser. No. 13,437 filed 2-21-79 both of which applications are assigned to the assignee of this application. The subject matter of these applications is hereby incorporated by this reference.
The selective leaching of magnesium values from spent oil shale retorts is known and described in the aforementioned related patent applications.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit having layers containing an organic polymer called "kerogen", which upon heating decomposes to produce hydrocarbon liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of method have been proposed for processing oil shale which involve either first mining the kerogen bearing shale and processing the shale above ground, or processing the oil shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
Although the process disclosed herein for selectively leaching magnesium values from combusted oil shale is primarily discussed in relation to leaching in an in situ oil shale retort, the process can be practiced on spent oil shale produced by other methods of retorting. Many of these methods for shale oil production are described in Synthetic Fuels Data Handbook, compiled by Dr. Thomas A. Hendrickson and published by Cameron Engineers, Inc., Denver, Colo. For example, other processes for retorting oil shale include those known as the Tosco, Paraho Direct, Paraho Indirect, N-T-U, Union Oil, and Bureau of Mines, Rock Springs, processes. Oil shale can also be processed by a combination of such techniques whereby a portion of the oil shale is excavated to create void space underground and retorted above ground, and another portion is explosively expanded toward such void spaces to form in situ retorts for underground retorting.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972 to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by this reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by mining out a portion of the subterranean formation and then fragmenting a portion of the remaining formation to form a stationary, fragmented peameable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen containing gaseous combustion zone feed to advance the combustion zone through the retort. In the combustion zone oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to product heat and combustion gas. The combustion zone is maintained at a temperature lower than the fusion temperature of oil shale, which is about 2100.degree. F., to avoid plugging of the retort, and above 1100.degree. F. for efficient recovery of hydrocarbon products from the oil shale.
The effluent gas from the combustion zone comprises combustion gas, carbon dioxide from mineral carbonate decomposition, and any gaseous portion of the combustion zone feed that does not take part in the combustion process. This effluent gas is essentially free of free oxygen and contains constituents such as oxides of carbon, water vapor, nitrogen, and sulfurous compounds. It passes through the fragmented mass in the retort on the advancing side of the combustion zone to heat oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and to a residue of solid carbonaceous material.
The liquid products and gaseous products are cooled by cooler particles in the fragmented mass in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, including shale oil, together with water produced in or added to the retort, are collected at the bottom of the retort and withdrawn to the surface through an access tunnel, drift or shaft. An effluent gas, referred to herein as off gas, containing combustion gas generated in the combustion zone, gaseous products including methane produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous portion of the combustion zone feed that does not take part in the combustion process is also withdrawn from the bottom of the retort.
Inorganic carbonates can be present in oil shale, notably carbonates of magnesium and calcium which decompose endothermically when heated to their decomposition temperatures. For example, oil shale particles in an in situ retort can contain approximately 8 to 12 weight percent calcium and 1.5 to 3 weight percent magnesium present as carbonates. Carbonate of magnesium is widely distributed in both dawsonitic and non-dawsonitic oil shales in the Piceance Creek Basin and can be a significant source of magnesia, given practical techniques for recovery of the magnesium values.
Magnesium carbonate can be present initially in the formation in a variety of mineral forms of varying composition, such as magnesite or brucite; in association with calcium carbonate as dolomite, a calcium magnesium carbonate; with iron as ferroan, an iron magnesium carbonate; and with calcium and iron as ankerite, a form of dolomite in which there is about 15 percent iron substituted for Magnesium. In stoichiometric dolomite, there is one magnesium atom per calcium atom. Calcium-rich dolomites having ratios of magnesium to calcium of less than one also occur. The aforementioned mineral forms, and others including illite, dawsonite, analcime, aragonite, calcite, quartz, potassium feldspar, sodium feldspar, nahcolite, siderite, pyrite, and fluorite, have been identified by x-ray diffraction analysis. The presence of such mineral forms in oil shale has been reported in W. Rob et al., "Mineral Profile of Oil Shales in Colorado Core Hole No. 1, Piceance Creek Basin, Colorado", Energy Resources of the Piceance Creek Basin, Colorado, D. Keith Murry, Ed. Rocky Mountain Association of Geologists, Denver, Colorado, pages 91-100, (19074) and E. Cook, "Thermal Analysis of Oil Shales", Quarterly of the Colorado School of Mines, Vol. 65, pages 133-140 (1970), the disclosures of which are incorporated herein by this reference.
U.S. Pat. No. 4,036,299 to Cha, et al, assigned to the assignee of the present application and incorporated herein by this reference, describes a method of recovering shale oil from oil shale in an in situ oil shale report in which a combustion zone is advanced through a fragmented permeable mass of formation particles containing oil shale and carbonates of magnesium and calcium. The patent discloses that the combustion zone is maintained at a temperature of from about 1100.degree. F. to about 1400.degree. F. (593.degree.-760.degree. C.), preferably from about 1200.degree. F. to about 1300.degree. F. (649.degree.-704.degree. C.), to obtain shale oil while avoiding excessive dilution of gaseous retorting products with carbon dioxide from decomposition of inorganic carbonates, notable calcium carbonate, in the oil shale.
Above-mentioned U.S. Pat. No. 3,661,423 to Garrett discloses briefly that mineral values can be leached from retorted oil shale in an in situ oil shale retort with water, acidic, or alkaline leaching agents. However, there is no description of the selective recovery of magnesium values from combusted oil shale in an in situ oil shale retort.
A number of patents have described the recovery of aluminum values from dawsonitic oil shale retorted above ground or by advancement of a combustion zone through an in situ oil shale retort, by leaching with aqueous leaching agents. Exemplary of these are U.S. Pat. Nos. 3,502,372 to Prats, 3,516,787 to Van Nordstrand, 3,572,838 to Templeton, 3,510,255 to Hall et al., and 3,642,433 to Dyni. The leaching agent typically is water or an alkaline aqueous solution, although the use of dilute acids has also been mentioned. The Van Norstrand patent states that oil shale can contain from about 10 to 40 weight percent dolomite, and that dolomite in oil shale is decomposed upon retorting to form carbon dioxide, calcite, and magnesium oxide, the magnesium oxide tying up part of the silica in the oil shale to permit higher recovery of the aluminum values by leaching. Recovery of magnesium values is not disclosed in these patents.
The recovery of magnesium values from ground, calcined dolomite, a mineral form of calcium magnesium carbonate, is known. The selective leaching of magnesium values from dolomite calcined at 750.degree.-850.degree. C. with carbonated water has been described, for example, in U.S. Department of the Interior, Bureau of Mines Technical Paper 684, "The Bicarbonate Process for the Production of Magnesium Oxide", by H. A. Doerner et al (1946), the disclosure of which is incorporated herein by this reference. This paper describes the leaching of magnesium values from slurries of finely ground, calcined dolomite in well agitated mixing tanks.
The thermal decomposition of carbonates of magnesium and calcium in oil shale is described in E. J. Jukkola et al, "Thermal Decomposition Rates of Carbonates in Oil Shale", Industrial and Engineering Chemistry, 45 (1953), 2711-2714, which is incorporated herein by this reference. Data obtained by heating oil shale over a range of temperatures under various partial pressures of carbon dioxide are reported. Leaching of magnesium values from retorted oil shale is not described.
U.S. Pat. No. 3,455,796 to Crumb relates to a method for treating residual material from oil shale retorting for recovery of its magnesium content to form magnesium chloride. Crushed oil shale residue is mixed with water and calcium hydroxide if needed to establish a pH of about 10 or higher. The resulting slurry comprises mineral hydrates including magnesium hydroxide, calcium hydroxide and finely divided unreacted particles. The separated slurry is introduced into a primary carbonation stage with a suitable source of carbon dioxide. The carbonation rate is slow enough to assure that calcium carbonate is formed with minimum formation of magnesium carbonate. The slurry is treated in a secondary carbonation stage with additional carbon dioxide and liquid containing magnesium chloride solution to increase calcium carbonate formation. Hydrochloric acid is introduced to neutralize the discharge from the second carbonation to a final pH of about 7.8. Acid is introduced at a slow rate so that magnesium hydrate dissolves without dissolving calcium carbonate to form a solution containing dissolved magnesium chloride.